The use of electrostatic probes as a diagnostic tool of the dust particles in the tokamak edge plasmas is investigated. Probe measurements of electrostatic fluctuations in the scrape-off layer of the Frascati Tokamak Upgrade revealed that some features of the signals can be explained only by a local non-propagating phenomenon. These signal features are shown to be both in qualitative and quantitative agreement with ionization, and consequent extra charge collected by the probes, due to the impact of micrometre-sized dust at a velocity of the order of 10 km s−1. Electron microscope analysis of the probe surface yielded direct support for such an interpretation.

In this work we describe the heating results in the LH frequency range (f = 2.45 GHz, Pw 5 250 kW, P < 6 k_W/cmz) both in the electron and ion regimes. Efficient electron heating was observes f o r n 5 5 ~1 0 ' ~ c w 3 . Increases of the peak electron temperature up to 700 eV were measured. At higher densities the interaction with electrons disappears. In the ion regime fast neutral tails and neutron enhancement were observed. The influence of plasma boundary conditions on the penetration of the wave is demonstrated. The principal physical problems are pointed out and some possible conclusions are given.

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A thorough investigation of confinement in Frascati Tokamak Upgrade has been carried out on a new database of ohmic, L-mode and advanced scenario discharges (multiple pellet-fuelled, radiation improved and internal transport barriers (ITBs)) obtained with the available auxiliary heating systems, namely electron cyclotron resonant heating, lower hybrid and ion Bernstein wave. A general agreement of the measured τ E with ITER97 L-mode scaling is found in ohmic and L-mode discharges. An improvement of the energy confinement time (τ E ) of up to about 60% over the ITER97 L-mode scaling has been obtained in ITB discharges, together with a reduction in local electron transport in the region of high pressure gradient, and up to about 30% in pellet-fuelled discharges (where τ E as large as ∼120 ms have been reached). The linear density dependence of τ E in ohmic discharges has been found to extend above the saturation density threshold in pellet-fuelled plasmas.

Plasma behaviour in the high density regime has been investigated on the Frascati Tokamak Upgrade (FTU). The items particularly addressed are density limit, MARFE characteristics and fuelling efficiency. With gas puffing, a maximum line average density of 3.2 x 10" m-3 has been obtained in the ohmic regime at q = 5.1, corresponding to 1.7 times the Greenwald limit; while with pellet injection, a value of 3.7 x 10' ' m-' has been reached at Q = 3.5. The density limit appears to be connected with the impurity content and edge parameters, so the best results are obtained with very clean plasmas and peaked electron density profiles. The MARFE phenomenon always appears beyond a critical density that depends on the total input power and the effective charge: emissivities in the range of 2 to 12 MW/m3 have been measured in this highly radiating region. The fuelling efficiency, starting from a value of about 50% at low density, progressively deteriorates, falling to 10% near the density limit: this behaviour is interpreted as being due to a decrease of the scrape-off layer transparency to incoming neutrals.

The energy confinement of plasma in FT (Frascati Tokamak) has been studied in a wide range of plasma currents (Ip up to 550 kA) and densities (n̄ up to 4.5 × 1014 cm−3). The Lawson parameter n̂τE has reached a value of 4 × 1013 cm−3 ·s. The main objective was to investigate the limits of validity of the linear relationship between confinement time and density. The ion heat flux has been found to be consistent with neoclassical ion transport. The electron heat diffusion coefficient has been found to increase with plasma current. Indeed, the electron thermal diffusion coefficient has been shown to be approximately proportional to the electron drift parameter (vD/vth). Thus the linear relationship of the electron energy confinement time with density holds only for discharges with the same current. – The deviation from a linear dependence of the totalenergy confinement time on density is due to ion transport and, therefore, appears to be a neoclassical effect.

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The scaling of the energy confinement time τE in the FT tokamak is discussed for discharges with Zeff = 1. Measurements are reported which confirm earlier FT results of a τE dependence on a positive power of the temperature and on the aspect ratio.